The present application relates to a thin film optical filter, and in particular to a thin film filter (TFF) sandwiched between a pair of substrates for rendering the TFF athermal or for providing a means by which the TFF can be actively tuned.
A conventional structure of a thin film interference filter (TFF) assembly 1 is illustrated in FIG. 1, in which a single layer or multiple thin film layers 2 are supported by a substrate 3. Adjacent thin film layers are firmly connected at the interfaces. The substrate 3 and each layer 2 are in a certain stress state based on the manufacturing history and the temperature distribution in the assembly 1. Some components, e.g. substrate 3, layers 2, may be in compression, while others are in tension. For each stress state, each layer 2 has a certain physical thickness and certain optical properties (refractive index, extinction). The stress state also determines the curvature of the surface and interfaces. All these parameters determine the optical response of the assembly 1 when light is incident thereon. When the temperature of the assembly 1 is changed all elements of the system move to a new stress state, which alters the physical thicknesses, the optical properties and the curvature of the system. Accordingly, the optical response of the whole system changes with temperature.
Typically, bandpass filters for light around 1550 nm are needed for telecommunication applications. One important requirement of such a filter is that the center wavelength remains within a specified range over the temperature range of operation. Haruo Takahashi published a paper entitled xe2x80x9cTemperature stability of thin-film narrow-bandpass filters produced by ion-assisted depositionxe2x80x9d in Applied Opics, Vol. 34, No 4, pp 667-675 on February 1995 relating to how to achieve that goal. The basic idea is to use a substrate with a high coefficient of thermal expansion (CTE). When the temperature of a bandpass filter rises, the thin film layers want to expand and the index of refraction goes up. Both of these effects lead to an increase in the optical thickness and a shifting of the filter towards longer wavelengths. However, if the TFF structure is supported by a substrate that has a higher CTE compared to the thin film materials, the substrate expands further than the thin films and thus stretches the films laterally. Due to the physical effects defined by Poisson""s ratio, a rise in temperature can thereby reduce the physical thickness of the TFF layers. The stress state of each layer also reacts to this stretching, therefore, if the right combination of substrate material, substrate thickness and thin film design is used, the system can be made athermal, i.e. the center wavelength remains almost constant over a wide range of temperatures, and active temperature control is not required.
For a device without active temperature control it can be assumed that the optical filter assembly 1 has a homogeneous temperature distribution at all times. The conventional structure acts like a bi-metal when the temperature is changed, whereby the substrate 3 expands or contracts more than the films 2. This leads to a change in curvature of the surface and all interfaces, which impacts the optical properties of the light incident on the filter, and limits how thin of a substrate can be used. Since the substrate is attached to only one side of the filter, not all of the layers are influenced equally by the substrate""s expansion, i.e. layers closer to the substrate are stretched more than layers remote therefrom. This disparity leads to gradients through the filter, which causes a curvature in the TFF and results in changes in the optical properties thereof, e.g. bandwidth narrowing.
More recent developments in this field, exemplified by U.S. Pat. No. 6,304,383 issued Oct. 16, 2001 to William Boynton et al, include a second stress applying member positioned on the opposite side of the TFF than the substrate. In these cases the second stress applying means is manufactured out of a vastly different material than the substrate, and requires a central channel therethrough to enable the light to pass. Unfortunately, opposite faces of the TFF still undergo different degrees of stretching, due to the use of a different material on each face. The Boynton patent also discloses an active TFF assembly in which electrostrictive or magnetostrictive layers are provided in the filter for actuation by an electric or magnetic source.
An object of the present invention is to overcome the shortcomings of the prior art by providing a passive athermalized TFF assembly in which a TFF is sandwiched between two similar substrates.
Another object of the present invention is to provide an actively tunable TFF assembly in which a TFF is sandwiched between two substrates, which can place the TFF under stress using controllable means.
Accordingly, the present invention relates to a thin film filter assembly comprising:
a thin film filter for passing a first signal defined by a first center wavelength through a first side and out from a second side thereof, and for reflecting a second signal defined by a second center wavelength out from the first side thereof;
a substrate mounted on the first side of the thin film filter for supporting the thin film filter thereon, the substrate applying a first force which varies with temperature; and
a superstrate mounted on the second side of the thin film filter for applying a second force to the thin film filter which varies with temperature, the second force being substantially equal to the first force;
whereby the first force and the second force combine to minimize curvature of the assembly and to minimize a shift in the first or second center wavelength caused by a change in temperature thereof.
Another aspect of the present invention relates to a tunable thin film optical filter assembly comprising:
a thin film filter for filtering an optical signal incident thereon;
a substrate mounted on the first side of the thin film filter for supporting the thin film filter thereon, and for applying a first force to the thin film filter which varies with temperature;
a first stress applying means for actively controlling an extra amount of stress applied to the first side of the thin film filter;
a superstrate mounted on the second side of the thin film filter for applying a second force to the thin film filter which varies with temperature;
a second stress applying means for actively controlling an extra amount of stress applied to the second side of the thin film filter;
whereby stress is independently applied to the substrate or the superstrate for tuning a characteristic of a response of the thin film filter.
Another feature of the present invention provides a variable attenuator assembly comprising:
an input waveguide for launching an input beam of light comprising a first signal and a second signal;
a first lens for collimating the input beam of light;
a thin film filter for reflecting a first signal out from a first side thereof, and for passing the second signal through the first side and out from a second side thereof;
a substrate mounted on the first side of the thin film filter for supporting the thin film filter thereon, and for applying a first force to the thin film filter which varies with temperature;
a first stress applying means for actively controlling an extra amount of stress applied to the first side of the thin film filter;
a superstrate mounted on the second side of the thin film filter for applying a second force to the thin film filter which varies with temperature;
a second stress applying means for actively controlling an extra amount of stress applied to the second side of the thin film filter;
an output waveguide for outputting the first signal; and
a second lens optically coupled to the thin film filter for focusing the first signal onto the output waveguide;
wherein the thin film filter has a standard curvature for optimally optically coupling the input waveguide to the output waveguide via the first and second lenses; and
wherein the first and second stress applying means change the curvature of the thin film filter, thereby attenuating the first signal.